1. Field of the Invention
The invention relates to a double-balanced mixer and, in particular, to a double-balanced mixer with a dynamic current steering cell.
2. Description of the Related Art
Mixer circuits for high frequency application constructed using metal oxide semiconductor (MOS) transistors are subject to a limited voltage supply (usually less than 2V) and high levels of flicker noise, having frequencies extending up to several tens of MHz. Accordingly, the gain and output signal levels required in such mixer circuits exceed those allowed in the equivalent bipolar circuits.
FIG. 1 is a circuit diagram illustrating a conventional double-balanced mixer circuit. The double-balanced mixer circuit in FIG. 1 includes differential pairs of MOSFETs (Q131-Q132 and Q133-Q134). The drains of the pairs of MOSFETs are connected to an output terminal (Output-I+ and Output-I−). The gates of the pairs of MOSFETs are connected to first input terminals (Input-II+ and Input-II−). The double-balanced mixer circuit in FIG. 1 also includes active devices Q135, Q136, Q137 and Q138. The sources of the MOSFET pair Q131-Q132 are connected to the drains of the active devices Q135 and Q136. The sources of the MOSFET pair Q133-Q134 are connected to the drains of the active devices Q137 and Q138. The gates of the active devices Q135, Q136, Q137 and Q138 are connected to the second input terminal (Input-I+ and Input-I−). The sources of the active devices Q135, Q136, Q137 and Q138 are connected to the ground through an impedance unit (Degeneration Impedance). A quadrature modulator (QMOD) configured with two conventional double-balanced mixer circuits in FIG. 1 in quadrature modulates baseband I/Q signals onto a carrier and generates modulated RF signals. As is known, in a direct-conversion transmitter the carrier signal which is of same frequency as the RF can leak to the output terminal of the QMOD. This carrier-leakage is a result of the DC-offset of I/Q input signals, device mismatches as well as through direct substrate coupling. Bit-error rate (BER) of the QMOD can be degraded unless a specific carrier-suppression requirement is met.
FIG. 2 is a circuit diagram of a conventional programmable mixer as disclosed in U.S. Pat. No. 6,801,761. As shown in FIG. 2, the conventional programmable mixer comprises first and second mixing stages 130 and 132, a coupling element 136 coupled therebetween, and a compensation module 134 coupled to the first and second mixing stages 130 and 132. The compensation module provides current mode compensation to the first and second mixing stages 130 and 132. Current from mixing stages 130 and 132 is substantially equalized by the addition of compensation current using current-sink or through injection via current-source to subtract current from the mixing stages 103 and 132.